Post reference: https://powershell.org/forums/topic/custom-class-5-dscresources-share-methods-between-dsc-classes/

powershell-forum-custom-class-5-dscresources-share-methods-between-dsc-classes-reply-01.ps1

[DscResource()]
class SampleResourceStandalone {
    [DscProperty(Key)]
    [string] $Name

    # Gets the resource's current state.
    [SampleResourceStandalone] Get() {

        return $this
    }

    # Sets the desired state of the resource.
    [void] Set() {
        Invoke-SampleResourceHelper
    }

    # Tests if the resource is in the desired state.
    [bool] Test() {
        return $false
    }
}

[DscResource()]
class SampleResourceCluster {
    [DscProperty(Key)]
    [string] $Name

    # Gets the resource's current state.
    [SampleResourceCluster] Get() {

        return $this
    }

    # Sets the desired state of the resource.
    [void] Set() {
        Invoke-SampleResourceHelper
    }

    # Tests if the resource is in the desired state.
    [bool] Test() {
        return $false
    }
}
function Invoke-SampleResourceHelper {
    # Do work
    Write-Verbose -Message 'Do work ...'
    Start-Sleep -Seconds 20
    Write-Verbose -Message '... done!'

}